Prisms

Prisms
are triangular pieces of glass or plastic. As light enters a
prism it is refracted, and then it refracts again when departing.
Before we look more closely at prisms, let us examine a piece of
glass or plastic which has parallel sides.

In our study under Refraction, we found that the light ray
going into an object with parallel sides exits going in the same
direction. It will be displaced to one side or the other, but
Snell's Law predicts the two angles are equal.

n1 sin q1
= n2 sin q2

n2 sin q3
= n1 sin q4

n1 sin q1
= n2 sin q2
= n2 sin q3
= n1 sin q4

n1 sin q1
= n1 sin q4

q1 =
q4

And the same phenomenon occurs whether the medium inside the
"box" is more or less dense than the surrounding material.

With non-parallel sides, we don't expect a prism to behave in
exactly the same manner. The light going into the prism is
refracted closer to the normal, making qP smaller than qA.

The angle formed at the next surface, q'P, is not equal to qP.
This means that the next equation, although simply a new
application of Snell's Law, does not contain the same size angle
as we saw in the case of parallel sides. (It is possible that they
would be the same sized angle, but generally not highly likely.)

A prism bends the light two times, having a cumulative effect
of bending it away from its original direction. The general
direction of bending is towards the wide portion or the base of
the prism.

What would happen if the prism were air and the surroundings
were more dense?

With a less dense "prism," the initial bending is away from the
normal making the angle inside the air larger than the angle
outside the air.

The second bending is from a larger angle in air to a smaller
angle in the new medium.

The cumulative effect is to double-bend the light towards the
narrow part or the apex of the prism.

Future versions of these pages will explore the peculiar effect
of prisms which enables them to spread light out into its
component colors. For now, rest assured that the behavior of
light rays in prisms follows the general rules discussed before.